Method for directly bonding ceramic and metal members and laminated body of the same

ABSTRACT

A laminated body comprising a ceramic member and a metal member, and a method of forming the laminated body are described. The laminated body is characterized in that the ceramic member contains in its surface portion a bonding agent and the metal member is directly bonded to the surface of the ceramic member. The method of forming the laminated body is characterized in that a bonding agent-containing layer is first formed in the surface of the ceramic member and then the bonding agent-containing layer is heated while being contacted with the metal member.

This is a division of application Ser. No. 697,874 filed Feb. 4, 1985,now U.S. Pat. No. 4,693,409 which in turn is a divisional of applicationof Ser. No. 507,004 filed June 23, 1983, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a method for directly bonding a metalmember to a ceramic member, and to a laminated body of ceramic and metalmembers obtained by this method.

A conventional method for bonding a metal member to a ceramic member ofalumina or the like is known in which a molybdenum paste is baked on thesurface of the ceramic member, and nickel plating is then performed tosecurely bond the metal member thereto. This method is adopted when theceramic involved is an oxide. However, when the ceramic is made of asubstance other than an oxide, such as a nitride, another method isadopted. According to this method, a recessed area is formed in one ofthe members to be bonded, while a projection is formed on the othermember in a corresponding pattern, and the two members are then shrinkfitted to each other. The former method is complex in procedure andrequires two heat treatment steps. On the other hand, the latter methodcan provide only a weak adhesive strength since the two members arebonded only by a clamping force.

To solve this problem, still another method has been proposed in which ametal member is directly bonded to a ceramic member. This methodincludes a first method in which a metal member is bought into directcontact with a ceramic member and the two members are heated in a gasatmosphere containing a bonding agent; and, a second method in which ametal member is surface-treated with a bonding agent, the treated metalmember is brought into contact with a ceramic member and the two membersare heated in an inert gas atmosphere (e.g., U.S. Pat. No. 3,994,430).

However, these methods have a common disadvantage in that, when theceramic member consists of a ceramic which does not contain a bondingagent at all or contains such agent only in a small amount, bonding ofsuch a ceramic member to a metal member is very difficult.

This means that if a ceramic member consists of a nonoxide-type ceramic,such as silicon nitride, and the bonding agent is oxygen, bonding of theceramic member to a metal member will be difficult. The reasons for thisare considered to be attributable to the facts that a eutectic of ametal and a bonding agent which is produced at the interface between theceramic member and the metal member has poor wettability with a ceramicmember consisting of a nonoxide-type ceramic, and that oxygen containedin a ceramic member contributes in some manner to producing a stablecompound with a metal.

SUMMARY OF THE INVENTION

The present invention has been made in view of this situation and hasfor its object to provide a method for directly bonding a ceramic memberto a metal member even if the ceramic member consists of a ceramic whichdoes not contain a bonding agent, such as a nonoxide-type ceramic.

It is another object of the present invention to provide a laminatedbody in which a ceramic member and a metal member are directly bonded toeach other, such as a circuit board with excellent heat conductivity, oran envelope for an electron tube or a rectifying element.

In accordance with an aspect of the present invention, a method isprovided for directly bonding a ceramic member and a metal member,comprising the steps of forming, in at least a surface layer of theceramic member, a layer containing a bonding agent in an amountsufficient to bond the ceramic member to the metal member, and heatingthis layer while it is in contact with the metal member.

In accordance with another aspect of the present invention, a laminatedbody which consists of a ceramic member and a metal member is alsoprovided, comprising a ceramic member having, in a surface layerthereof, a layer containing a bonding agent in an amount sufficient tobond the ceramic member to the metal member, and a metal member which isdirectly bonded to the layer.

Detailed Description of the Preferred Embodiments

The present invention is based upon a finding that, if the surface of aceramic member is surface-treated with a bonding agent or is bondedafter a bonding agent is added thereto, the ceramic and metal membersmay be securely bonded to each other in such a way that they will notseparate over a long period of time.

The ceramic member to be used herein may consist of a nonoxide-typeceramic such as a nitride, e.g., silicon nitride, aluminum nitride, ortitanium nitride; a carbide, e.g., silicon carbide or titanium carbide;a boride, e.g., lanthanum boride; or, an oxide-type ceramic such assilica, alumina and zirconia.

A ceramic selected may contain, as a sintering auxiliary, an oxide ofyttrium, aluminum, calcium, strontium or beryllium; a carbonate; orother salts.

The method of the present invention is particularly effective in bondinga nonoxide-type ceramic member to a metal member.

A bonding agent to be used herein produces a eutectic in combinationwith a metal and may include oxygen, sulfur, phosphorus, silicon or thelike. The bonding agent may be selected in accordance with the types andcombinations of the ceramic and metal members. For example, when themetal member comprises copper, iron, chromium or the like, the mostdesirable bonding agent is oxygen or sulfur. If the metal membercomprises aluminum, silicon is suitable for use as a bonding agent.

The amount of bonding agent needed to achieve satisfactory bonding issomewhere between the maximum amount at which the bonding agent is inthe form of a solid solution and the eutectic-forming amount.

Addition of a bonding agent to the surface layer of the ceramic membermay be performed by a conventional method. For example, when a ceramicmember consists of a nitride and the bonding agent is oxygen, theceramic member is heated to 1,000° to 1,400° C. in air or is heated to1,250° to 1,500° C. in a wet-forming gas (H₂ +N₂), to achieve oxidation.When the heating temperature is lower than the above-mentioned ranges, arequired bonding agent layer is not formed. On the other hand, when theheating temperature is higher than these ranges, the formed bondinglayer becomes separated from the ceramic member. Heating in air allowsformation of a bonding agent layer (oxide layer) at a low temperature,while heating in a wet-forming gas allows formation of a dense bondingagent layer (oxide layer). The bonding agent layer formed in this mannerpreferably has a thickness of about 20 μand more preferably 10 μ orless. The effect of formation of a bonding agent layer may be obtainedwith a layer thickness of about 0.001 μ or more.

When the bonding agent is contained throughout the ceramic member, thebonding agent may be contained in the form of a compound. For example,when the bonding agent is oxygen, it may be contained in a nonoxide-typeceramic member in the form of an oxide.

The oxide may be yttrium oxide, titanium oxide, alumina, silica,magnesium oxide, or a glass. The oxide as the bonding agent may be addedin an amount of 5 to 50% by weight, and preferably 10 to 30% by weight.When the amount of the oxide is less than the lower limit, bondingstrength becomes unsatisfactory. On the other hand, when the amount ofthe oxide exceeds the upper limit, the original characteristics of theceramic are impaired. Particularly preferable results may be obtained ifthe oxide is used in the amount of 10 to 30% by weight.

When sulfur is used as the bonding agent, it may be contained in theform of a sulfide. When phosphorus is used as the bonding agent, it maybe contained in the form of a glass containing phosphorus. When siliconis used as the bonding agent, the metal is molybdenum and the ceramic isalumina or aluminum nitride, which may be contained in the form of MoSi₂. However, when silicon is used as the bonding agent, the metal isaluminum and the ceramic is alumina or aluminum nitride, which may becontained in the form of AlSi₂.

The metal member to be used in the present invention may consist of asimple body of copper, iron, chromium, nickel, molybdenum, silver,cobalt, or aluminum; or, alloys or mixtures thereof. The shape of themetal member may be columnar, plate-like, foil-like, or granular.

The metal member preferably contains a bonding agent at least in asurface layer thereof. For example, when the metal member consists ofcopper, a metal containing a bonding agent (preferably oxygen) in theamount of 80 to 3,900 ppm is preferably used. This is to allow easybonding. When the metal is copper and the bonding agent is oxygen, anelectrolytic tough pitch copper is preferably used. Bonding may also befacilitated if the metal is surface-treated with a bonding agent to forma surface layer of 0.01 μm to 1 μm containing the bonding agent.

To bond metal and ceramic members together, the metal member is broughtinto direct contact with the surface of a ceramic member which containsa bonding agent or with the surface of a ceramic member which has beensurface-treated (by a bonding agent impregnation treatment process). Theheating temperature is preferably below the melting point of the metaland above the eutectic temperature of the eutectic. For example, whenthe metal member consists of copper and the bonding agent is oxygen, theheating temperature is below the melting temperature of copper (1,083°C.) and above the eutectic temperature of copper-copper oxide (1,065°C.). When a metal member contains a bonding agent or is surface-treatedtherewith, heating is performed in a gas atmosphere which is inert tothe ceramic member, such as a nitrogen atmosphere. When a metal memberdoes not contain a bonding agent, heating is performed in a reactiveatmosphere containing 0.03 to 0.1 vol % of a bonding agent, tofacilitate easy bonding. It is preferable for minimizing the deformationof a bonded structure to bond metal members on opposite surfaces of aceramic member.

EXAMPLE 1

A plate-shaped ceramic member was used which consisted of a siliconnitride containing 4% by weight of yttrium oxide and 4% by weight ofalumina as a sintering auxiliary, and which had dimensions of 30×30×2mm. The ceramic member was heated at 1,250° C. in air for 1 hour and theceramic surface was slightly oxidized. A plate-shaped metal memberconsisting of electrolytic tough pitch copper and having dimensions of10×50×0.3 mm (300 to 500 ppm oxygen content) was brought into contactwith the oxidized surface of the ceramic member. These members were thenheated at 1,075° C. in this state in a nitrogen atmosphere. After themembers were cooled to substantially room temperature, their bondingstate was examined, and proved to be excellent. The peel strength of themembers was measured by a peel test and was determined to be 5˜10 kg/cm.

COMPARATIVE EXAMPLE 1

The procedures of Example 1 were followed, except that the ceramicmember was not heated in air but was brought directly into contact witha metal member of an electrolytic tough pitch copper. The resultantlaminated body had a peel strength of 1˜3 kg/cm.

EXAMPLE 2

A plate-shaped ceramic member was used which consisted of an aluminumnitride containing 2% by weight of yttrium oxide and 2% by weight ofalumina as a sintering auxiliary, and which had dimensions of 30×30×2mm. The ceramic member was heated in air at 1,200° C. for 1 hour and theceramic surface was slightly oxidized. A plate-shaped metal memberconsisting of electrolytic tough pitch copper and having dimensions of10×50×0.3 mm was brought into contact with the oxidized surface of theceramic member. The two members were then heated at 1,075° C. in thisstate in a nitrogen atmosphere. After the members were cooled tosubstantially room temperature, their bonding state was examined, andproved to be excellent. The peel strength of the members was measured tobe 8˜12 kg/cm.

EXAMPLE 3

A plate-shaped ceramic member was used which consisted of a siliconcarbide containing 4% by weight of yttrium oxide and 4% by weight ofalumina as a sintering auxiliary, and had dimensions of 30×30×2 mm. Theceramic member was heated in air at 1,150° C. for 1 hour and the ceramicsurface was slightly oxidized. A plate-shaped metal member consisting ofan electrolytic tough pitch copper and having dimensions of 10×50×0.3 mmwas brought into contact with the oxidized surface of the ceramicmember. These members were then heated at 1,075° C. in this state in anitrogen atmosphere. After the members were cooled to a substantiallyambient temperature, their bonding state was examined, and proved to beexcellent. The peel strength of the members was measured to be 5˜10kg/cm.

EXAMPLE 4

A plate-shaped ceramic member was used which consisted of a siliconnitride containing 5% by weight of yttrium oxide, 5% by weight ofalumina and 5% by weight of silica, and which had dimensions of 30×30×2mm. A plate-shaped ceramic member of an electrolytic tough pitch copperand having dimensions of 10×50×0.3 mm was brought into contact with theceramic member. These members were then heated in this state in anitrogen atmosphere at 1,075° C. for 10 minutes. After the members werecooled to a substantially room temperature, they were examined forbonding strength. The peal strength of the resultant laminated body wasmeasured to be 5˜10 kg/cm.

COMPARATIVE EXAMPLE 2

A plate-shaped ceramic body was used which consisted of a siliconnitride, containing 3% by weight of magnesium oxide, and had dimensionsof 30×30×2 mm. A metal member consisting of an electrolytic tough pitchcopper and having dimensions of 10×50×0.3 mm was brought into contactwith the ceramic member. These members were heated in a manner similarto that in Example 3. The laminated body obtained had poor bondingstrength and its peel strength was measured to be 1˜3 kg/cm.

EXAMPLE 5

A plate-shaped ceramic member was used which consisted of aluminumnitride containing 5% by weight of yttrium oxide, 5% by weight ofalumina and 5% by weight of silica, and which had dimensions of 30×30×2mm. A metal member consisting of electrolytic tough pitch copper andhaving dimensions of 10×50×0.3 mm was brought into contact with theceramic member. These members were heated in a nitrogen gas atmosphereat 1,075° C. for 10 minutes. After the members were cooled tosubstantially room temperature, they exhibited excellent bondingstrength. The resultant laminated body had a peel strength of 8˜12kg/cm.

EXAMPLE 6

A plate-shaped ceramic member was used which consisted of a siliconnitride containing 5% by weight of yttrium oxide, 5% by weight ofalumina and 5% by weight of silica, and had dimensions of 30×30×2 mm. Ametal member consisting of an electrolytic tough pitch copper and havingdimensions of 10×50×0.3 mm was brought into contact with the ceramicmember. These members were heated in this state in a nitrogen gasatmosphere at 1,075° C. for 10 minutes. After the members were cooled toa substantially room temperature, they exhibited excellent bondingstrength. The resultant laminated body had a peel strength of 5˜10kg/cm.

In summary, according to the method of the present invention, even aceramic member which does not contain a bonding agent may be directlybonded to a metal member with ease. The method of the present inventionis useful in heat-conductive assemblies such as semiconductor mountsubstrates, envelopes for electron tubes or rectifying elements, etc. Inparticular, combinations of aluminum nitride/copper, and siliconcarbide/copper are preferable as heat-conductive assemblies.

What is claimed is:
 1. A heat conductive substrate comprising analuminum nitride ceramic plate having a surface oxidized at a thicknessof 20 microns or less and a copper layer directly bonded to saidoxidized surface, a eutectic crystal being formed between the copperlayer and said oxidized surface.
 2. A heat conductive substrateaccording to claim 1, wherein said aluminum nitride ceramic platecontains a sintering auxiliary.
 3. A heat conductive substrate accordingto claim 2, wherein said sintering auxiliary is selected from the groupconsisting of: oxides and carbonates of yttrium, aluminum, calcium,strontium or beryllium.